Through a photochemical reaction of nitrogen dioxide and 2-trans-butene, peroxyacetyl nitrate (PAN) is formed, which plays an important role in polluted air. The decomposition of this has been studied under two conditions: continued irradiation and in the absence of further irradiation. It was found that the rate of decomposition differed markedly in these two conditions. Different reaction mechanisms are postulated to account for this difference.
INGE FILD and D. J. LOVELL, "Infrared Spectral Studies of Peroxyacetyl Nitrate (PAN)," J. Opt. Soc. Am. 60, 1315-1318 (1970)